The CME predicts that \(\gamma \)  \(>\)  0 for opposite sign-pairs and \(\gamma \)  \(<\)  0
for same sign-pairs. There are other two particle azimuthal correlation effects
that can depend on the reaction plane driven by elliptic flow even though the
underlying correlation may be independent of the reaction plane. These
backgrounds are summarized in Ref.[1]}. It was pointed in
Ref.[1]} that the \(\phi \)  difference correlation (\(\delta \) ) which is
independent of the reaction plane gives a constraint on the CME and
backgrounds.
\(\delta = \langle cos(\phi _1 - \phi _2) \rangle = \langle cos(\phi _1) cos(\phi _2) \rangle + \langle sin(\phi _1) sin(\phi _2) \rangle .\) 
